(Not Applicable)
The invention generally relates to copolymer compositions having active oxygen scavenging capacity and the use of these compositions for improved packaging of oxygen sensitive substances. Improved formulations are disclosed which may be fabricated into packaging articles. Also disclosed are improved packaging article constructions and embodiments.
This application and the related applications, as recited above, are generally directed to active oxygen scavenging copolymers and their use in packaging articles. Most embodiments disclosed in this and the related applications involve the use of the active oxygen scavenging copolymers in a laminar composite as at least one of several layers comprising the wall or film of a packaging article.
The active oxygen scavenging copolymers of this and the related applications are typically copolycondensates comprising predominantly polycondensate segments and a lesser amount of oxygen scavenging moiety (OSM) segments. It is a common practice to use copolymers, more specifically copolycondensates, as packaging and bottling materials, For example, even common polyethylene terephthalate (PET) bottles used for bottling soda pop often comprise some isophthalic linkages in the polymer and thus could be called a copolymer. In order to avoid such ambiguities, the term oxygen scavenging copolymer or modified copolymer will be used to designate those polymers which have OSM segments and which are an important component of this invention. The oxygen scavenging modified copolymers of this invention are prepared as thermoplastics in that they are suitable for melt processing into bottles and packaging articles. However, applications for these materials are envisioned where the thermoplastic material may subsequently be transformed into a thermoset resin.
It is known that post polymerization treatment of modified packaging and bottling polycondensates (i.e., those which have not been modified so as to add oxygen scavenging ability) may generally improve the salient packaging characteristics of such materials. The post polymerization treatment may involve heat and/or chill treatment, chemical treatment/reaction, irradiation, aging, or combinations thereof. A non-exhaustive list of improvements which may be realized includes devolatization, drying, increased crystallization, further polymerization (especially solid state polymerization), increased intrinsic viscosity (IV), increased molecular weights, improved odor and/or taste, improved passive barrier properties, and improved clarity. The application of post polymerization treatment techniques to unmodified polycondensate to obtain such improvement is relatively well known in the art. Application of these post polymerization techniques to active oxygen scavenging copolymers in a manner similar to the techniques used on unmodified polycondensates has here-to-fore proven unsatisfactory because of problems encountered such as loss of oxygen scavenging capacity and discoloration or loss of clarity of the oxygen scavenging copolymers. What is needed are methods for post polymerization treatment of oxygen scavenging copolymers which provide the desired improvements gained for unmodified polycondensates but which avoid the problems previously encountered for post polymerization treatment of oxygen scavenging copolymers. This application addresses these and other needs leading to improved active oxygen scavenging copolymers and packaging articles made therefrom.
For more complete understanding of this and the related applications, it should be recognized that there are two broad types of barriers for shielding packaged oxygen sensitive substances from oxygen (generally oxygen from air). One is known as a passive oxygen barrier and finds utility because of superior resistance to the permeation of oxygen through such constructions. Glass and metal are essentially perfect passive oxygen barriers. Condensation polymers, especially polyesters such as polyethylene terephthalate (PET), have found wide acceptance in the packaging industry and are moderately good passive oxygen barriers. Other condensation polymers may exhibit either superior or inferior passive barrier properties than those of polyesters. For example, polyamides, such as polyhexamethylene adipamide and polyphthalamides, are generally better passive oxygen barriers than polyesters when deployed in similar constructions.
The other type of oxygen barrier is known as an active oxygen barrier. An active oxygen barrier is a substance capable of intercepting and scavenging oxygen (by undergoing chemical reaction with the oxygen), for example, as the oxygen attempts to permeate through the packaging. A major salient feature of active oxygen scavengers is their ability not only to intercept oxygen from air as it attempts to reach the package cavity but also to provide the means to eliminate unwanted oxygen (often called head space oxygen) from within the package cavity wherein said oxygen may have been inadvertently introduced during packaging or filling. Only active oxygen scavengers can remove unwanted oxygen from the package cavity. Active oxygen scavenging implies, therefore, transformation of a material incorporated in the package as it reacts with and depletes oxygen. The material is progressively consumed so that the active oxygen scavenging ability is eventually depleted or at least diminished. However, this eventual depletion of the active oxygen scavenging moiety can be adjusted so that the depletion occurs only well after the required oxygen free shelf life of the packaged product which is typically one year or less.
The active oxygen scavenging copolymers of this and the related applications are typically copolycondensates comprising predominantly polycondensate segments and a lesser amount of oxygen scavenging moiety (OSM) segments. Predominantly, as used above, means over 50 wt % of the copolymer is comprised of polycondensate segments. Comprising as used in this application is defined as xe2x80x9cspecifying the presence of stated features, integers, steps, or components as recited, but not precluding the presence or addition of one or more other steps, components, or groups thereofxe2x80x9d. Comprising is different from xe2x80x9cconsisting ofxe2x80x9d which does preclude the presence or addition of one or more other steps, components, or groups thereof. The polycondensate segments of the oxygen copolymers are typically polyester (especially comprising PET) or polyamide, but polysulfones, polyethers, polyketones and other polycondensates are also envisioned as sources of polycondensate segments for the oxygen scavenging copolymers of this application. The OSM segments of the oxygen scavenging copolymers are typically polyolefin oligomers, polypropylene oxide oligomers, or methyl pendant aromatic compounds as defined in the related application having Application Number PCT/US98/05239. The wt % of OSM segments employed in the copolymers is usually in the range of about 0.5 to 12 wt % and preferably in the range of about 2 to about 8 wt %. The copolymers may be produced by any means but are typically made by placing terminal functional group(s) on the OSM which are capable of entering into polycondensation, transesterification, transmigration, and similar transfer reactions. The required amount of these functionalized OSM""s are then used as a xe2x80x9cpseudo monomerxe2x80x9d in a batch or continuous polycondensation or reacted with a previously prepared polycondensate where they are incorporated into the polymer by transesterification thus producing the active oxygen scavenging copolymer.
In selected applications, such as for packaging non-comestible products such as electronic components, the oxygen scavenging copolymers may be deployed as a single layer of film to form the packaging article. More often, the oxygen scavenging copolymer comprises at least one layer of a multi-layered packaging article. In many embodiments, at least one of the other layers comprises a packaging polycondensate such as a polyester or polyamide. The packaging polycondensate layer may be comprised of a mixture of monomers which may add salient packaging features to the layer. For example, in a layer of packaging polyester, it is typical to find PET having some linkages present other than ethylene and terephthalate. Typically the PET layer would contain some isophthalate and/or some naphthalate linkages, for example. These same linkages may also be present in the polyester blocks of the copolycondensate when the oxygen scavenging copolymer is a copolyester.
The oxygen scavenging copolymers are often used with and/or further comprise materials which improve their packaging properties and/or which enhance their active oxygen scavenging ability. Ideally, the following characteristics are considered to be desirable for the oxygen scavenging copolymers:
(1). Glass transition temperatures above about 60xc2x0 C. are preferred so that the copolymers exist as solids at ambient temperatures of about 0 to 60xc2x0 C.
(2). Intrinsic viscosity (IV) of at least about 0.5 is preferred for easier processing into bottles and/or films.
(3). Good clarity is preferred for clear packaging applications.
(4). Commercially acceptable scavenging capacity of at least 0.4 cc of O2 per gram of copolymer at ambient temperatures is preferred to ensure adequate shelf-life of packaged product with minimal use of copolymer.
(5). Low OSM segment percentages (2 to 8 wt %) are preferred so that used packaging articles may be more readily recycled and so that the modified polymer is similar in properties to the unmodified polycondensate from which the polycondensate segments were derived.
(6). Good passive barrier properties are preferred not only to keep out O2 but also to retain CO2 in packaging of carbonated beverages.
The selected use of these enhancing materials includes pyromellitic dianhydride (PMDA), transition metal catalysts (preferably cobalt), photo-active substances such a benzophenone, oxidation moderators or controllers such as butylated hydroxy toluene (BHT), and others. Applicants have further determined that cobalt derived from cobalt carboxylates is most effective. Cobalt from cobalt octoate or cobalt stearate is preferred, and cobalt from cobalt octoate is especially preferred. The enhancing materials may be deployed individually or in various combinations. These enhancing material additives may comprise a portion of the copolymer composition during formation of the copolymer, may be added later to either the modified polymer or the packaging article, or some of the enhancing materials may be added by each of the methods.
The oxygen scavenging modified polymers are typically used as layers in a multi-layered wall/film packaging article. An especially preferred embodiment comprises use of the copolymer in conjunction with layers of polycondensate from which the polycondensate segments in the copolymer were derived.
It is known that post polymerization treatment of packaging and bottling polycondensates may generally improve the salient packaging characteristics of such materials. The post polymerization treatment may involve heat and/or chill treatment, chemical treatment/reaction, irradiation, aging, or combinations thereof. A non-exhaustive list of improvements which may be realized includes devolatization, drying, increased crystallization, improved (higher temperature) melt strength, further polymerization (especially solid state polymerization), increased intrinsic viscosity (IV), increased molecular weights, improved odor and/or taste properties, improved passive barrier properties, and improved clarity. The application of post polymerization treatment techniques to unmodified polycondensate to obtain such improvement is relatively well known in the art. Application of these post polymerization techniques to active oxygen scavenging copolymers in a manner similar to the techniques used on unmodified polycondensates has here-to-fore proven unsatisfactory because of problems encountered such as loss of oxygen scavenging capacity and discoloration or loss of clarity of the oxygen scavenging copolymers.
An early patent which discloses development of advantageous packaging properties for PET by post polymerization treatment is U.S. Pat. No. 3,553,157 of Dijkstra et al. The Dijkstra patent discloses both heat treatment and chemical treatment of PET. One chemical treating agent exemplified, among several disclosed in the Dijkstra patent, is PMDA. In a preferred embodiment of this application, Applicants disclose oxygen scavenging copolymer formulations comprising PMDA which is added during formation of the copolymer or is already present in a pre-formulated concentrate mix and serves as a chain extending agent. While there may be some residual PMDA remaining from its use during formation of the oxygen scavenging modified polymers of this invention, those of ordinary skill in the art will recognize that formulation of PMDA into the copolymer during synthesis is different from post polymerization treatment with PMDA. This is especially the case since PMDA is preferred because it essentially reacts to completion. Regardless, there is no disclosure what-so-ever in the Dijkstra patent regarding post polymerization treatment of oxygen scavenging copolyesters.
Another patent, U.S. Pat. No. 4,145,466 of Leslie et al. discloses the use of PMDA and PMDA derivatives in the post polymerization processing of PET primarily for improvement of melt strength of the PET. Again, there is no disclosure what-so-ever in this reference regarding post polymerization treatment of oxygen scavenging copolyesters. Two other patents, U.S. Pat. No. 5,243,020 and U.S. Pat. No. 5,338,808 both by Guido Ghilsolfi, describe the post polymerization upgrading of PET and PET-like polyesters but do not disclose methods for post polymerization upgrading of active oxygen scavenging copolyesters.
The active oxygen scavenging copolycondensates of this and the related applications are comprised predominantly of polycondensate segments and a lesser amount of OSM segments. When placed in the walls of packaging materials such as bottles, cups, films, trays, etc., oxygen attack on the packaged contents may be eliminated or at least held to acceptable levels for the required shelf life of the product. The capacity to absorb oxygen and the rate of such oxygen scavenging are key performance characteristics of the oxygen scavenging copolycondensates. The copolycondensates can be placed into container walls by a variety of melt processing methods as was disclosed in PCT Application PCT/US97/16711 published on Mar. 26, 1998 having International Publication Number WO 98/12127. The oxygen scavenging copolycondensates typically undergo post polymerization treatment prior to fabrication into containers to facilitate such processing and to improve the performance of the fabricated article. The oxygen scavenging modified polymers of this invention may also undergo post polymerization treatment after fabrication into containers/packages. These operations include, among others, drying, devolatization, crystallization, and solid state polymerization. In general, they are similar to those typically applied to non-oxygen scavenging homopolymers and copolymers. However, the application of such post polymerization techniques to oxygen scavenging copolymers here-to-fore has substantially diminished the oxygen scavenging capacity of such copolymers. One key feature of this invention was the identification and implementation of critical changes in these operations such that the oxygen scavenging capacity of the copolymers was not significantly diminished. An important concept of this invention was the discovery and perfection of methods to perform such operations in the strict absence of oxygen and to limit the copolymer oxygen exposure after such operations.